1. Field of the Invention
The present invention relates to a thermal conductivity detector and method for operating the thermal conductivity detector.
2. Description of the Related Art
Thermal conductivity detectors are used to detect certain liquid or gaseous substances (fluids) based on their characteristic thermal conductivity, particularly in gas chromatography. Here, components or substances of a gas mixture are separated by passing a sample of a gas mixture in a carrier gas (mobile phase) through a separation column containing a stationary phase. The different components interact with the stationary phase that causes each component to elute at a different time, which is known as the retention time of the component. The separated substances, also referred to as analytes, are detected by a thermal conductivity detector which has a measuring cell with an appropriate detector element, e.g., an electrically heated filament disposed in a measurement channel. Depending on the thermal conductivity of the substance flowing past the heated filament, more or less heat is diverted from the heating filament to the wall of the measurement channel, and the heating filament is correspondingly cooled to a greater or lesser degree. As a result of the cooling of the heating filament, its electrical resistance changes, which is detected.
For this purpose and as known from, e.g., U.S. Pat. No. 5,756,878, the heating filament may be disposed in a measuring bridge, which contains additional resistors. The thermal conductivity of the substance passing the heating filament is obtained from an amount of energy that is supplied to the measuring bridge and controlled to maintain the temperature of the heating filament at a predetermined operating temperature. Instead of the resistors, further filaments may be provided that are fluidically parallel or in series with the filaments in the measurement channel and a reference channel, respectively.
From U.S. Pat. No. 5,379,630 or U.S. Pat. No. 5,587,520, it is known to provide two resistors and a controllable switch in one arm of a measuring bridge. Here, the switch is controlled to periodically change the resistance of the arm -between the values of the two resistors and thus alternately operate the heating filament at two different temperatures. The thermal conductivity of a substance flowing past the heated filament is determined from the difference of power dissipated by the filament at the two different operating temperatures.
U.S. Pat. No. 3,733,463 discloses a temperature control system that includes a modified Wheatstone bridge with a resistive-capacitive (RC) circuit in one arm of the bridge. The RC circuit includes a resistor that provides an effective resistance as a function of its absolute resistance and the on-time to off-time ratio of pulses supplied to a switch connected thereacross. A sawtooth voltage is produced across the RC circuit, where the voltage is compared with the voltage across a temperature sensor, and where heat is applied during each pulse period portion when the sawtooth voltage exceeds the voltage across the temperature sensor. As the Wheatstone bridge is powered from a DC battery, the operating temperature of the temperature sensor remains completely unaffected by the resistive-capacitive (RC) circuit.
The operating temperature of the heating filament is set by the ratios of the resistances in the measuring bridge. As the resistances are temperature dependent, it can be advantageous to use integrated, rather than discrete, resistors in a monolithic device. This provides the most stable measurement due to the cancellation of temperature effects, while the resistors are on the same substrate. However, a disadvantage of such a monolithic device is that manufacturing variances of the resistors do not provide a strong certainty of the exact operating temperature of the heating filament, or at the very least, a match in temperatures between successive thermal conductivity detectors, such as one in a measurement channel and another in a reference channel. Therefore, for practical use in, e.g., a gas chromatograph, the manufacturing variation of the monolithic devices requires adjustment of the resistance ratios.
Any effort to insert a variable resistor (potentiometer) or a compensating resistance is undesirable, both from a manufacturing perspective as well as from the potential addition of undesired noise and drift. A more advanced method would be laser trimming of the resistances. However, this is impractical for a micromachined (MEMS) device including the measurement channel with the filament, since the channel must be enclosed about the resistance elements, thus also closed to a laser trimming operation. The trimming operation would have to occur in an intermediate manufacturing step of the monolithic device, thus adding cost and value earlier in the process, thus running a higher risk of costly yield failures.
The thermal conductivity detector used in a gas chromatograph provides an output signal that represents a quantitative time domain spectrum of the composition of the gas mixture as a series of peaks (chromatogram). Each peak represents a component of the gas mixture, where the height and area of the peak determine the quantity of the component. The peaks can be very small and yet, within the same chromatogram, some peaks may be very large. For further digital processing, the chromatogram must be digitized, preferably with a high resolution of, e.g., 24 bit.
The quality of the measurement mainly reduces the signal to noise ratio of the complete measurement system as well as the resolution that is available in the analog to digital converter (ADC). It is obvious that in order to account for the largest peak, the resolution of the smaller peaks will be compromised. If, for example, a large peak is resolved with 100 steps, a 10 times smaller peak will be resolved with only 10 steps. Clearly the 100 step measurement is better defined than the 10 step measurement.
Thus, it might appear obvious to provide a variable electronic gain amplifier that increases the gain on the output signal of the thermal conductivity detector during periods where small peaks are expected. However, noise and wander effects are a significant challenge when performing chromatography and the variable gain amplifier would also amplify the noise and wander of the baseline of the chromatogram and thus only propagate the uncertainty of the measurement. Variable gain electronic systems also have a tendency to add noise and wander within the very bandwidth of the low frequency chromatogram. The chromatographic peak itself is Gaussian shaped, and the fidelity of this shape is important. Classic filtering of a noisy chromatogram, however, would distort the Gaussian shape.